The present invention relates to an exhaust nozzle for a gas turbine engine and in particular to an exhaust nozzle capable of vectoring a flow of exhaust gases issuing therefrom at an angle to the centre line of the engine.
Military aircraft often need to have a high rate of turn in situations where the aircraft control surfaces are inadequate. For this purpose it is desirable for the engine to be provided with a propulsion nozzle which can deflect the exhaust gases to give a thrust at an angle to the centre line of the engine.
A conventional exhaust nozzle for thrust vectoring comprises a convergent and divergent section. The convergent section of the nozzle defines at its downstream end a nozzle throat of variable diameter. The exhaust gases are accelerated through the nozzle throat to the divergent section which contains the high pressure gas generated by the accelerated exhaust gases. The divergent portion defines at its downstream end a nozzle exit. The convergent and divergent sections of the nozzle comprise a plurality of radially outer and radially inner flaps which are circumferentially disposed around the engine. The plurality of flaps are substantially rectangular and rigid and define a flow path through which the exhaust gases flow. The flow path for the exhaust gases is defined by the outer and inner flaps alternately overlapping each other. The outer flaps in the convergent and divergent sections of the nozzle are actuated positively to vary the nozzle throat and exit respectively. The inner flaps stay in contact with the outer flaps due to the outward gas pressure of the exhaust gases when the engine is operational. When the engine is not operational the inner flaps are kept in contact with the outer flaps by flexible spring retainers or other similar devices.
Actuation of the outer flaps may be provided by a number of actuators which act on the outer flaps either independently or via a unison ring. The actuators can act directly on the outer flaps or through an outer fairing. To obtain deflected thrust the outer flaps of the divergent section of the nozzle are actuated so as to move them asymmetrically with respect to the engine centre line. The outer flaps are moved differentially so that the outer flaps on one side of the nozzle move to a different angle than the outer flaps on the other side. In this way the thrust line can be moved up or down or side to side to control the pitch and yaw axis.
A problem with conventional convergent/divergent nozzles is that when the outer flaps in the divergent section are moved asymmetrically to take up different angles, the inner flaps are unable to stay in contact with adjacent outer flaps. The inner flaps are therefore unable to seal against the outer flaps and leakage of the exhaust gases can occur.